As used herein, the term tachycardia refers to any fast abnormal rhythm of the heart which may be amenable to treatment by electrical discharges and specifically includes supraventricular tachycardia (SVT), atrial fibrillation and flutter (AF), ventricular tachycardia (VT), and ventricular flutter and ventricular fibrillation (VF).
Rubin's U.S. Pat. No. 3,857,398, dated Dec. 31, 1974, and entitled "Electrical Cardiac Defibrillator", describes a combined pacer/defibrillator. This device either performs a bradycardia pacing or a defibrillation function depending on the detection of a VT/VF. If a VT/VF is detected, the device is switched to the defibrillating mode. After a period of time to charge the capacitor, a defibrillation shock is delivered to the patient.
Improvements to this device were contained in a multiprogrammable, telemetric, implantable defibrillator which is disclosed in Gilli et al. copending patent application Ser. No. 239,624, filed Sep. 1, 1988, and entitled "Reconfirmation Prior to Shock in Implantable Defibrillator". The Gilli et al. device contains a bradycardia support system as well as a high energy shock system to revert ventricular tachycardias to normal sinus rhythm. On reconfirmation of the presence of a tachycardia, a shock is delivered to the patient at a predetermined time or when the desired energy level is reached.
As cardioversion or defibrillation shocks can be very unpleasant to a patient, especially when delivered frequently, it became necessary therefore to provide a device which included antitachycardia pacing therapy along with bradycardia support pacing therapy and defibrillation or cardioversion therapy, so that the implanted device could automatically provide the necessary therapy from a range of therapies offered by the device. Hence a further development in the field of combined implantable devices is described in U.S. Pat. No. 4,940,054, invented by Grevis and Gilli, dated Jul. 10, 1990, and entitled "Apparatus and Method for Controlling Multiple Sensitivities in Arrhythmia Control Systems Including Post Therapy Pacing Delay". This device is a microcomputer based arrhythmia control system which is programmable by means of a telemetric link. The device provides single chamber bradycardia support pacing, antitachycardia pacing, and cardioversion or defibrillation shocks for restoring normal sinus rhythm to a patient.
Additionally, various specific developments have been made in the field of tachycardia control pacers. Tachycardia is a condition in which the heart beats very rapidly, with a ventricular rate higher than 100 bpm and typically above 150 bpm, and an atrial rate as high as 400 bpm. There are several different pacing modalities which have been suggested for the termination of tachycardia. The underlying principle in all of them is that if a pacer stimulates the heart at least once shortly after a heartbeat, before the next naturally occurring heartbeat at the rapid rate, the heart may successfully revert to normal sinus rhythm. Tachycardia is often the result of electrical feedback within the heart. A natural beat results in the feedback of an electrical stimulus which prematurely triggers another beat. By interposing a stimulated heartbeat, the stability of the feedback loop is disrupted.
Langer et al., in U.S. Pat. No. 4,202,340, entitled "Method and Apparatus for Monitoring Heart Activity, Detecting Abnormalities, and Cardioverting a Malfunctioning Heart", describes an antitachycardia pacing system which detects VT/VF by deriving a probability density function from the analysis of the morphology or shape of intracardiac signals. This antitachycardia pacing system is subject to errors in the delivery of therapy due to the erratic and unpredictable nature of intracardiac signals.
The system disclosed in U.S. Pat. No. 4,475,551, entitled "Arrhythmia Detection and Defibrillation System and Method", invented by Langer et al. and issued on Oct. 9, 1984, illustrates arrhythmia detection in which the device first analyzes the probability density function to ascertain abnormal cardiac rhythms such as fibrillation, high rate tachycardias, and low rate tachycardias. Upon the discovery of such rhythms, the device senses heart rate so as to distinguish fibrillation and high rate tachycardia from low rate tachycardia. This device employs a predetermined threshold value for the rate which distinguishes such arrhythmia events. The inclusion of rate criteria to define arrhythmias was unable to remedy the deficiencies inherent in the probability density function operation; therefore the trend of later devices was to perform arrhythmia detection on the basis of rate information alone.
Geddes, in U.S. Pat. No. 4,291,699, entitled "Method and Apparatus for Automatically Detecting and Treating Ventricular Fibrillation" and issued on Sep. 29, 1981, characterizes a defibrillator which senses both the electrical and mechanical activity of the heart to detect fibrillation. This device measures the mechanical pumping action of the heart by detecting changes in electrical impedance between a pair of electrodes implanted within one of the ventricles of the heart. The diagnostic relevance of the impedance measurement requires an accurate assessment of electrical conduction volume within the heart. In devices which measure impedance using only two electrodes, gross volume approximation errors occur which lead to large inaccuracies in the impedance measurement. Furthermore, the diagnostic utility of the impedance measurement is degraded by extraneous influences on the impedance signal such as noise from respiration, changes in the patient's posture, and electrical interference.
A major disadvantage inherent in devices which detect cardiac arrhythmias on a basis of rate criteria alone is their propensity to inappropriately deliver shocks during physiological tachyarrhythmias or nonmalignant pathological tachyarrhythmias. In such devices, the rate standard cannot distinguish ventricular tachycardia or ventricular fibrillation from physiological tachycardia (also termed sinus tachycardia, herein) which normally arises from stress or exercise conditions and does not require antitachycardia pacing or defibrillation shock therapy. In addition, the rate criteria cannot differentiate VT or VF from the fast ventricular responses resulting from other pathological tachycardias such as supraventricular tachycardia or atrial flutter and fibrillation. Arrhythmia detectors using a rate basis are frequently confused by line frequency noise.
The use of antitachycardia pacing therapy in conjunction with a dual chamber pacing device is disclosed in the copending application of Norma L. Gilli, Ser. No. 462,499, filed Jan. 5, 1990, and entitled "Apparatus and Method for Antitachycardia Pacing in Dual Chamber Arrhythmia Control System", which application is assigned to the assignee of the present invention. In the Gilli application, upon detection of the presence of tachycardia, the tachycardia cycle length (TCL) is measured and a V-A interval less than or equal to the TCL is determined, along with an initial value A-V interval. Stimulation pulses are then delivered until the expiration of a given number (N) of V-A intervals and N A-V intervals, to complete a first train of pulses. A series of a given number (M) of trains of pulses similar to the first train of pulses is delivered, and the A-V delay interval is varied from the initial value thereof at least once prior to the completion of the series of M trains of pulses. Monitoring of intrinsic QRS complexes between pulse trains is performed. If the tachyarrhythmia is deemed to be accelerating, one operation of cardioversion or defibrillation is employed. The present invention is an improvement over said Gilli application with respect to the manner of detecting tachyarrhythmias.